The invention relates to a process for the thermal bonding of fibers to produce nonwoven fabrics with bonding or binder powder wherein the nonwoven fabric is produced from textile fibers including reclaimed wool, cotton, or spun rayon, or from synthetic fibers, such as acrylic, glass, or other mineral fibers and the fibers are uniformly mixed, during, for example, aerodynamic formation of a nonwoven mat or web, with binder powder of a phenolic resin or novolak, and wherein heated air is passed through the nonwoven mat or web for bonding purposes.
Fibrous mats of this type are essentially manufactured as blown fleeces by the aerodynamic method and are mixed during this production with bonding powders so that the latter are uniformly distributed in the resulting nonwoven fabric. For the bonding of mats or fleeces, so-called twin-belt ovens are utilized wherein the nonwoven mats or fleeces are held between two plate-shaped belts and are subjected to hot air or, respectively, are slightly ventilated. The problem in this bonding process resides in that the binder powder sticks to the platform belts and thus contaminates these belts. The binder powder is baked into the plates in the long run and it then becomes difficult to clean the dirt off the plates. The second problem resides in that the binder powder distributed in the nonwoven fabric is partially entrained by the air, even if the throughflow of air is merely slight, and is then blown about in the entire oven by turbulence. It is readily evident that in such a case the binder powder will settle on the walls and thus, in the course of time, will clog the entire oven with its stickiness. Furthermore, the binder particles entrained by the air are, of course, taken away from the nonwoven fabric so that the binder concentration remaining in the nonwoven fabric cannot be accurately defined. The proportion of binder powder, however, is essential for the properties of the nonwoven fabric to be obtained. Consequently, the properties of the nonwoven fabrics can be only conditionally predetermined.
In the previously known manufacturing method, gases are generated in the oven by the combustion of the binder components; these gases are toxic and therefore, also for this reason, must not be permitted to enter into the operator's cubicle. It is thus necessary to operate the platform conveyor oven with a great amount of waste air so that fresh air is taken in at the inlet and outlet and, consequently, no vapors escape therefrom. On account of this high quantity of waste air, the energy balance of the oven is, however, considerably impaired since the ovens normally are operated at a circulating air temperature of 200.degree. C., and thus the waste air likewise has this temperature. Since the phenolic resin vapors, however, are deleterious to health, the waste air must be passed through an after-burning unit. The afterburning unit normally operates at temperatures of 800.degree.-900.degree. C. It is readily apparent that the energy consumption in the conventional bonding method in the afterburning unit is even higher than the energy consumption in the oven for bonding the products.
Additionally, these twin-plate belt ovens work with a poor degree of efficiency since the two metallic plate belts carry a very large amount of energy to the outside. This is due to the fact that at least the lower platform belt must be additionally passed through a subsequent cooling zone for the nonwoven fabric. Thus, with each revolution, the belts must again be reheated.